A well maintained engine in the '30's, '40's could be expected to last for 40k miles. I almost bought a one owner '36 5 win in 1951 that had 31,k on the odometer, the car was garaged during it's whole life sitting in a heated parking garage. The father of a friend of mine had maintained the car since it was new for the local Ford Dealer. Mr Merrell talked me out of buying the car because it was on it's third engine, the result of low usage.
The '36 that I still own had 51.k on the odometer when I bought it in September 1952. it ran very well, however when the weather got cold in January the car did not want to start, poor compression. I had a valve job done, $35.00. I pulled the engine in April of '53, with 100,K showing on the odometer. I installed a well built 49AB engine, sold the original engine to a friend for his '36 Ford sedan work car. The engine ran for another six years before my friend junked the car.
The moral to my story; in the good ole days engines did not last like they do now.